Auto merge of #105323 - cjgillot:simplify-const-prop, r=davidtwco

Perform SimplifyLocals before ConstProp.

MIR before `ConstProp` may have a lot of dead writes, this makes `ConstProp` do unnecessary work.

r? `@ghost`

Author: bors

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -71,7 +71,7 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
         bbs[block].statements[statement_index].make_nop();
     }
 
-    crate::simplify::SimplifyLocals.run_pass(tcx, body)
+    crate::simplify::simplify_locals(body, tcx)
 }
 
 pub struct DeadStoreElimination;

compiler/rustc_mir_transform/src/lib.rs

@@ -557,6 +557,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &multiple_return_terminators::MultipleReturnTerminators,
             &instcombine::InstCombine,
             &separate_const_switch::SeparateConstSwitch,
+            &simplify::SimplifyLocals::new("before-const-prop"),
             //
             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
             &const_prop::ConstProp,
@@ -575,7 +576,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
             &o1(simplify::SimplifyCfg::new("final")),
             &nrvo::RenameReturnPlace,
-            &simplify::SimplifyLocals,
+            &simplify::SimplifyLocals::new("final"),
             &multiple_return_terminators::MultipleReturnTerminators,
             &deduplicate_blocks::DeduplicateBlocks,
             // Some cleanup necessary at least for LLVM and potentially other codegen backends.

compiler/rustc_mir_transform/src/simplify.rs

@@ -379,9 +379,21 @@ fn save_unreachable_coverage(
     ));
 }
 
-pub struct SimplifyLocals;
+pub struct SimplifyLocals {
+    label: String,
+}
+
+impl SimplifyLocals {
+    pub fn new(label: &str) -> SimplifyLocals {
+        SimplifyLocals { label: format!("SimplifyLocals-{}", label) }
+    }
+}
 
 impl<'tcx> MirPass<'tcx> for SimplifyLocals {
+    fn name(&self) -> &str {
+        &self.label
+    }
+
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
         sess.mir_opt_level() > 0
     }
@@ -557,6 +569,7 @@ fn remove_unused_definitions(used_locals: &mut UsedLocals, body: &mut Body<'_>)
 
                     StatementKind::SetDiscriminant { ref place, .. }
                     | StatementKind::Deinit(ref place) => used_locals.is_used(place.local),
+                    StatementKind::Nop => false,
                     _ => true,
                 };
 

src/test/codegen/zst-offset.rs

@@ -15,7 +15,7 @@ pub fn helper(_: usize) {
 pub fn scalar_layout(s: &(u64, ())) {
 // CHECK: getelementptr i8, {{.+}}, [[USIZE]] 8
     let x = &s.1;
-    &x; // keep variable in an alloca
+    witness(&x); // keep variable in an alloca
 }
 
 // Check that we correctly generate a GEP for a ZST that is not included in ScalarPair layout
@@ -24,7 +24,7 @@ pub fn scalar_layout(s: &(u64, ())) {
 pub fn scalarpair_layout(s: &(u64, u32, ())) {
 // CHECK: getelementptr i8, {{.+}}, [[USIZE]] 12
     let x = &s.2;
-    &x; // keep variable in an alloca
+    witness(&x); // keep variable in an alloca
 }
 
 #[repr(simd)]
@@ -36,5 +36,8 @@ pub struct U64x4(u64, u64, u64, u64);
 pub fn vector_layout(s: &(U64x4, ())) {
 // CHECK: getelementptr i8, {{.+}}, [[USIZE]] 32
     let x = &s.1;
-    &x; // keep variable in an alloca
+    witness(&x); // keep variable in an alloca
 }
+
+#[inline(never)]
+fn witness(_: &impl Sized) {}

src/test/incremental/hashes/closure_expressions.rs

@@ -42,9 +42,9 @@ pub fn add_parameter() {
 }
 
 #[cfg(not(any(cfail1,cfail4)))]
-#[rustc_clean(cfg="cfail2", except="hir_owner_nodes, optimized_mir, typeck")]
+#[rustc_clean(cfg="cfail2", except="hir_owner_nodes, typeck")]
 #[rustc_clean(cfg="cfail3")]
-#[rustc_clean(cfg="cfail5", except="hir_owner_nodes, optimized_mir, typeck")]
+#[rustc_clean(cfg="cfail5", except="hir_owner_nodes, typeck")]
 #[rustc_clean(cfg="cfail6")]
 pub fn add_parameter() {
     let x = 0u32;

src/test/mir-opt/const_allocation.main.ConstProp.after.32bit.mir

@@ -10,12 +10,12 @@ fn main() -> () {
         StorageLive(_2);                 // scope 0 at $DIR/const_allocation.rs:+1:5: +1:8
         _2 = const {alloc1: &&[(Option<i32>, &[&str])]}; // scope 0 at $DIR/const_allocation.rs:+1:5: +1:8
                                          // mir::Constant
-                                         // + span: $DIR/const_allocation.rs:8:5: 8:8
+                                         // + span: $DIR/const_allocation.rs:9:5: 9:8
                                          // + literal: Const { ty: &&[(Option<i32>, &[&str])], val: Value(Scalar(alloc1)) }
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation.rs:+1:5: +1:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation.rs:+1:8: +1:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation.rs:+1:8: +1:9
-        nop;                             // scope 0 at $DIR/const_allocation.rs:+0:11: +2:2
+        _0 = const ();                   // scope 0 at $DIR/const_allocation.rs:+0:11: +2:2
         return;                          // scope 0 at $DIR/const_allocation.rs:+2:2: +2:2
     }
 }

src/test/mir-opt/const_allocation.main.ConstProp.after.64bit.mir

@@ -10,12 +10,12 @@ fn main() -> () {
         StorageLive(_2);                 // scope 0 at $DIR/const_allocation.rs:+1:5: +1:8
         _2 = const {alloc1: &&[(Option<i32>, &[&str])]}; // scope 0 at $DIR/const_allocation.rs:+1:5: +1:8
                                          // mir::Constant
-                                         // + span: $DIR/const_allocation.rs:8:5: 8:8
+                                         // + span: $DIR/const_allocation.rs:9:5: 9:8
                                          // + literal: Const { ty: &&[(Option<i32>, &[&str])], val: Value(Scalar(alloc1)) }
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation.rs:+1:5: +1:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation.rs:+1:8: +1:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation.rs:+1:8: +1:9
-        nop;                             // scope 0 at $DIR/const_allocation.rs:+0:11: +2:2
+        _0 = const ();                   // scope 0 at $DIR/const_allocation.rs:+0:11: +2:2
         return;                          // scope 0 at $DIR/const_allocation.rs:+2:2: +2:2
     }
 }

src/test/mir-opt/const_allocation.rs

@@ -1,3 +1,4 @@
+// unit-test: ConstProp
 // ignore-endian-big
 // EMIT_MIR_FOR_EACH_BIT_WIDTH
 static FOO: &[(Option<i32>, &[&str])] =

src/test/mir-opt/const_allocation2.main.ConstProp.after.32bit.mir

@@ -10,12 +10,12 @@ fn main() -> () {
         StorageLive(_2);                 // scope 0 at $DIR/const_allocation2.rs:+1:5: +1:8
         _2 = const {alloc1: &&[(Option<i32>, &[&u8])]}; // scope 0 at $DIR/const_allocation2.rs:+1:5: +1:8
                                          // mir::Constant
-                                         // + span: $DIR/const_allocation2.rs:5:5: 5:8
+                                         // + span: $DIR/const_allocation2.rs:6:5: 6:8
                                          // + literal: Const { ty: &&[(Option<i32>, &[&u8])], val: Value(Scalar(alloc1)) }
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation2.rs:+1:5: +1:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation2.rs:+1:8: +1:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation2.rs:+1:8: +1:9
-        nop;                             // scope 0 at $DIR/const_allocation2.rs:+0:11: +2:2
+        _0 = const ();                   // scope 0 at $DIR/const_allocation2.rs:+0:11: +2:2
         return;                          // scope 0 at $DIR/const_allocation2.rs:+2:2: +2:2
     }
 }

src/test/mir-opt/const_allocation2.main.ConstProp.after.64bit.mir

@@ -10,12 +10,12 @@ fn main() -> () {
         StorageLive(_2);                 // scope 0 at $DIR/const_allocation2.rs:+1:5: +1:8
         _2 = const {alloc1: &&[(Option<i32>, &[&u8])]}; // scope 0 at $DIR/const_allocation2.rs:+1:5: +1:8
                                          // mir::Constant
-                                         // + span: $DIR/const_allocation2.rs:5:5: 5:8
+                                         // + span: $DIR/const_allocation2.rs:6:5: 6:8
                                          // + literal: Const { ty: &&[(Option<i32>, &[&u8])], val: Value(Scalar(alloc1)) }
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation2.rs:+1:5: +1:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation2.rs:+1:8: +1:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation2.rs:+1:8: +1:9
-        nop;                             // scope 0 at $DIR/const_allocation2.rs:+0:11: +2:2
+        _0 = const ();                   // scope 0 at $DIR/const_allocation2.rs:+0:11: +2:2
         return;                          // scope 0 at $DIR/const_allocation2.rs:+2:2: +2:2
     }
 }